1. Field of the Invention
This invention relates generally to auto tuning apparatus and more particularly to an auto tuning apparatus for use with a synthesizer tuner.
2. Description of the Prior Art
When automatically scanning tuning points using a so-called synthesizer tuner, a scan button, for example, in the synthesizer tuner is depressed. Then, the value of N, a positive integer, in a frequency-dividing ratio 1/N of a variable frequency-dividing circuit in a local oscillating circuit of the synthesizer tuner is varied to change the local oscillation frequency, so that the received frequency is changed. When a tuning point, i.e., a receiving frequency coinciding with the frequency of a wave broadcast from a given station, is detected, the changing of the value of N is stopped, thus stopping the scan and enabling the synthesizer tuner to receive a program of that broadcast station.
In this case, the tuning point, i.e., the receiving frequency coinciding with the frequency of the broadcast wave, is discriminated by detecting the intermediate frequency carrier.
For example, an intermediate frequency amplifying circuit rectifies an intermediate frequency carrier component derived from an intermediate frequency filter to produce a DC voltage. When the DC voltage exceeds a threshold level, that tuning point is detected as a correct or optimum tuning point, i.e., the receiving frequency coinciding with the frequency of the broadcast wave.
Various methods are known for detecting an intermediate frequency carrier: A method involving A/D (analog-to-digital) conversion of the rectified output; a method involving detection of a carrier frequency by using a counter; a method involving frequency-discrimination of a carrier frequency to detect it as the central value of its S-shaped characteristic; and so on.
In other cases, including the present case, the value of N is varied with a frequency interval corresponding to the precision with which the tuning point is to be detected. Generally, the value of N is varied in such a fashion that an FM tuner for, for example, the Japanese market can check the received signal at a frequency interval of 100 kHz, while an FM tuner for the European market or the like can check the received signal at a frequency interval of 50 kHz.
The preceding examples apply particularly to an FM tuner. In an AM tuner, the value of N is changed to provide detection at a frequency interval of, for example, 9 kHz.
There is a time delay when the tuning point, i.e., the receiving frequency coinciding with the frequency of 9 broadcast wave, is detected in a conventional manner by detecting an intermediate frequency carrier (this technique is disclosed in U.S. Pat. No. 4,298,989). As a result, when the scanning speed is too high, the detection of the tuning point cannot accurately follow the scan. Consequently, in most cases, the scan is stopped at a value of N that is one step beyond the value of N that gives the optimum or correct tuning point. In such cases, therefore, the correct or optimum tuning point can be obtained only by returning the value of N to a previous value (generally one step previous).
The frequency interval mentioned above, which corresponds to steps of varying the value of N, is narrower than the width of the passband of the intermediate frequency filter employed. Therefore, there may occur the case that, within the width of the passband of the intermediate frequency filter, there are contained a plurality of values of N, e.g., three such values NA, NO and NB, as shown in FIG. 1.
Thus, particularly when the received electric field intensity is high, there is a risk that, although the correct tuning point corresponds to an N-value of NO in FIG. 1, NA or NB respectively before and after NO will be falsely detected as the optimum or correct tuning point for the broadcast wave.
In such a case, it is sufficient that the width of the passband of the intermediate frequency filter be selected to encompass an odd number of values such as three and that the central value, i.e., the middle tuning point in the odd number of tuning points, be selected as the correct tuning point.
This method is, however, possible only on the condition that the intermediate frequency filter has a band-pass characteristic that is symmetrical with respect to its intermediate frequency.
A commercial band-pass filter, which is mass-produced, is not always free from the displacement of its center passband frequency. In other words, it often lacks symmetry with respect to an intermediate frequency. With the above-mentioned band-pass filter, there can occur a case such that an even number of potential tuning points, such as two or four, exist within the passband, with the result that a central tuning point cannot be identified, thus making it impossible in conventional practice to determine the correct or optimum tuning point.
Even if the intermediate frequency filter can initially maintain the symmetrical property described above, the sideband component is varied depending on the contents of the received signal, thus sometimes resulting in an even number of tuning points. Further, a filter with a wide band characteristic where more than five values of N are contained within its intermediate filter band characteristic is poor in selectivity. It is therefore customary to avoid the use of such a filter. The shortcomings described above cannot be avoided by the apparatus of the prior art.